1. Technical Field
The present invention relates to a laminated wave plate and an optical pickup device using the same.
2. Related Art
In optical pickup devices recording and regenerating an optical recording medium such as an optical disk including a compact disc (CD) and a digital versatile disc (DVD) or a magnet optical disc, a wave plate corresponding to a plurality of wavelengths is used because compatibility between CD and DVD is required.
JP-A-2001-209963 as a first example discloses a wave plate having a phase difference of 1625 nm. A relation between the phase difference of 1625 nm and laser light having a wavelength of 650 nm is expressed as 1625 nm=325 nm+650 nm×2. The relation between 325 nm and 650 nm is practically expressed as 325 nm/650 nm×2π=π in radian. Thus the wave plate generates a phase difference of π. In addition, a relation between the phase difference of 1625 nm and laser light having a wavelength of 790 nm is expressed as 1625 nm=835 nm+790 nm×1. The relation between 835 nm and 790 nm is practically expressed as 835 nm/790 nm×2π≈2π. Thus the wave plate generates a phase difference of 2π. Further, the first example discloses a wave plate having a phase difference of 1950 nm. A relation between the phase difference of 1950 nm and laser light having a wavelength of 650 nm is expressed as 1950 nm=650 nm+650 nm×2. The relation between 650 nm and 650 nm is practically expressed as 650 nm/650 nm×2π=2π in radian. Thus the wave plate generates a phase difference of 2π. In addition, a relation between the phase difference of 1950 nm and laser light having a wavelength of 790 nm is expressed as 1950 nm=370 nm+790 nm×2. The relation between 370 nm and 790 nm is practically expressed as 370 nm/790 nm×2π≈π in radian. Thus the wave plate generates a phase difference of π.
JP-A-2002-14228 as a second example discloses a phase element. The phase element is structured such that two transparent substrates are layered in a manner interposing a birefringent organic thin film therebetween. A phase difference with respect to laser light having a wavelength of 650 nm is expressed as 2π(m1−½). Thus a phase difference of π is substantively generated. A phase difference with respect to laser light having a wavelength of 790 nm is expressed as 2πm2. Thus a phase difference of 2π is generated.
JP-A-2002-250815 as a third example discloses a laminated wave plate. The laminated wave plate of the third example is structured such that a first wave plate (made of a birefringent organic material) having a phase difference of 593 nm (=¾×790 nm) and a second wave plate (made of a birefringent organic material) having a phase difference of 395 nm (=½×790 nm) with respect to light in a wavelength band of 790 nm are laminated in a manner intersecting their optical axes by 24°. The laminated wave plate rotates a polarization plane of linearly polarized light having a wavelength of 660 nm by 45° and converts linearly polarized light having a wavelength of 790 nm to circularly polarized light.
Further, WO 2003/91768 as a fourth example discloses the following laminated wave plate. The laminated wave plate of the fourth example is structured such that a first wave plate having a phase difference of 2700° (=180°+360°×7, that is, a seventh mode 180°. The substantive phase difference is 180°.) and a second wave plate having a phase difference of 630° (=270°+360°×1, that is, a first mode 270°. The substantive phase difference is 270°.) with respect to a wavelength of 655 nm are laminated in a manner intersecting their optical axes by 45°. The laminated wave plate functions as a quarter wave plate with respect to light having a wavelength of 655 nm and functions as a half wave plate with respect to light having a wavelength of 785 nm. Further, the fourth example discloses the following laminated wave plate. The laminated wave plate is structured such that a first wave plate having a phase difference of 2700° (=180°+360°×7, that is, a seventh mode 180°. The substantive phase difference is 180°.) and a second wave plate having a phase difference of 1260° (=180°+360°×3, that is, a third mode 180°. The substantive phase difference is 180°.) with respect to a wavelength of 655 nm are laminated in a manner intersecting their optical axes by 45°. The laminated wave plate functions as a half wave plate with respect to light having a wavelength of 655 nm and functions as a 2/2 wave plate with respect to light having a wavelength of 785 nm.
However, a phase difference of a wave plate is a function of a wavelength, so as to have such wavelength dependency that if a wavelength changes, a phase difference also changes. Therefore, in the wave plates disclosed in the first to third examples, a phase difference generated in laser light largely changes disadvantageously.
That is, since the phase difference has wavelength dependency, it largely changes if a wavelength shifts from a predetermined value as shown in FIG. 12 showing a relation of a wavelength and a phase difference. For example, this wave plate is mounted on an optical pickup device. If a wavelength of laser light emitted from a light source is shifted due to temperature drift and the like generated at the light source of a semiconductor laser that is used, for example, a phase difference generated in the laser light transmitting through the wave plate largely changes.
The laminated wave plate disclosed in the fourth example has a phase difference compensation function that compensates a change of a phase difference generated in laser light which transmits through the laminated wave plate even if a wavelength emitted from the light source is shifted, whereby the wavelength dependency is improved. However, the laminated wave plate cannot rotate a polarization plane of linearly polarized laser light being incident on the laminated wave plate by 90° disadvantageously.
A case where laser light is incident on a laminated wave plate 61 in which a first wave plate 62 and a second wave plate 63 are laminated as shown in FIGS. 13A and 13B will be described with Poincare sphere shown in FIG. 13C.
As shown in FIGS. 13A and 13B, the laminated wave plate 61 is structured such that the first wave plate 62 disposed at an incident side of the laser light and the second wave plate 63 disposed at an emitting side of the laser light are laminated in a manner intersecting an optical axis 62a of the first wave plate 62 and an optical axis 63a of the second wave plate 63 by an intersecting angle of 45° (=57°−12°).
Referring to FIG. 13C, when a linearly polarized light 64 incident from a point P0 on the equator transmits through the first wave plate 62, a phase difference of 180°+360°×7 is generated, so that the light 64 rotates by 180°+360°×7 around an axis R1 as a rotation axis to reach a point P1. Further, when the light 64 transmits through the second wave plate 63, a phase difference of 180°+360°×3 is generated, so that the light 64 rotates by 180°+360°×3 around an axis R2 as a rotation axis to reach a point P2. The point P2 is positioned on the equator of the Poincare sphere, and the phase difference generated when the linearly polarized light 64 transmits through the first wave plate 62 and the second wave plate 63 is substantively 180°. However, a polarization plane of the linearly polarized light 65 emitted from the laminated wave plate 61 is rotated by approximately 120° with respect to a polarization plane of the linearly polarized light 64. That is, a polarization plane of linearly polarized light that is incident is not orthogonal to a polarization plane of linearly polarized light that is emitted. Therefore, the laminated wave plate is not suitable to a case where p-polarized light is converted into s-polarized light or an optical system (optical use) in which abnormal light is required to be converted into normal light.
Therefore a wave plate described as follows is required to be realized. The wave plate can compensate variation of phase difference generated in the wave plate, on which laser light is incident, so as to keep the variation to be minimum even when the wavelength of the laser light emitted from a light source is shifted due to temperature drift and the like generated at the light source, and can rotate a polarization plane of linearly polarized light by 90° in a case where the plate functions as a half wave plate.
Such requirement is imposed not only on an optical element corresponding to two types of wavelengths having a wavelength range of around 785 nm and a wavelength range of around 655 nm in an optical pickup device compatible between CD and DVD, but also on an optical element corresponding to three types of wavelengths having a wavelength range of around 785 nm for CD, a wavelength range of around 655 nm for DVD, and a wavelength range of around 405 nm for Blu-ray, HDDVD, or the like.